A small cathode ray tube is used in an electronic view finder (EVF) of a portable VTR (so-called cam corder) or in a portable TV reciever, A cathode ray tube for these purposes requires super compactness, low power consumption, quick start-up, and the like, because such a portable VTR or TV uses batteries as the power source.
The cathode which emits electrodes in a small cathode ray tube is usually a direct heating type. A direct heating cathode structure includes an electrode emitter installed at the middle of a heater made of a metal wire or a metal piece. The cathode is operated so that the emitted electrons are focused in a beam by an electrostatic lens including one or more of grids or electrodes. The electron beams thus focused are scanned onto a phosphorescent screen to proper images.
However, the heater can be heated by the self-generated heat to produce a thermal deformation, with the result that the distance between the electron emitter and the grids or electrodes forming electrostatic lens are varied. If the emitter is displaced toward the electrodes, then the time required for operationally stabilizing the cathode is increased, and the cut-off characteristics are lowered, with the result that the electron emissions can not be stabilized.
Accordingly, various techniques have been proposed in order to prevent drifting of the emitter due to the thermal deformation of the heater. An example is described in Japanese Utility Model Publication No. Sho 60-3481.
According to this method, both ends of a heater on which an emitter is disposed are connected to an elastic body in order to tension the heater to inhibit thermal deformation during the operation of the heater. However, such a method has various problems as described below. For example, the heater receives a tensile stress, and therefore its dimensions are increased, with the result that power consumption is increased, and the heater is easily degraded due to fatigue, thereby shortening the life expectancy of the cathode. Further, the heater structure is very complicated, raising the manufacturing cost, and the heat loss is also enormous.
Another attempt for overcoming the disadvantages of the previous techniques is disclosed in Japanese Published Patent Application 59-184431. According to this method, a metal piece is formed in an approximate W shape as shown in FIG. 1. This metal piece is secured to a terminal 6 which is supported by a supporter 7, thereby forming a heater 2', with an electronic emitter 1 placed at the middle of the heater 2'.
This W shaped cathode structure above compensates structurally for thermal expansion as shown in FIG. 2 by moving to the position shown by the broken lines from the cold position shown by the solid lines when the heater 2' produces heat, so that the drifting of the emitter 1 from position 3 to position 3' is inhibited.
However, in this method, the expansion amounts are compensated simply based on the difference of the expansion directions of the different parts, and therefore, the drift inhibiting effect for the emitter is not sufficient. Further, a severe springing-back phenomenon occurs when the metal piece is bent into a W shape of heater, with the result that the rate of defective products is very high, reducing yield. Further, the bent portion indicated by reference numeral 4' in the drawing is a sharp point where heat is concentrated and, therefore, a considerable amount of heat is dissipated without being used in heating the emitter, with the result that the power loss is very large, and the period of time from first supplying power until electron emission is very lengthy, so that a quick start-up, is not expected. Further, the above mentioned bent portion 4' is a region where not only heat is concentrated, but also thermal stress and fatigue occurs most intensely due to the repetitions of the expansions and contractions caused by the heat concentration. Therefore this portion 4' is quickly damaged, shortening the life of the cathode.